Vacuum cleaners have a motor which typically drives an impeller to create a flow of air. The travelling head of the vacuum cleaner has an opening in its bottom wall through which air can enter the travelling head, the air carrying dirt and debris into the travelling head. It is arranged that the air transports the dirt and debris by way of ducts within the travelling head, the ducts typically having a cross-sectional area measuring around 7 to 10 cm2. The dirt and debris is transported through the ducts to a dirt-collection chamber. The air then passes through one or more filters before leaving the vacuum cleaner, the filters being arranged to trap the dirt and debris within the dirt-collection chamber for subsequent disposal.
The dirt-collection chamber can contain or comprise a disposable bag, the wall of the bag also acting as a filter. Alternatively, the dirt-collection chamber is a receptacle which can be removed from the vacuum cleaner, emptied, and re-installed into the vacuum cleaner for re-use.
Many vacuum cleaners have a rotatable brush located adjacent to the opening of the travelling head. The brush is rotated and engages the surface which is being cleaned. The brush helps to dislodge dirt and debris from the surface which is then gathered into the air flow and transported to the dirt-collection chamber.
A disadvantage of traditional vacuum cleaners is that some of the dirt and debris which has been dislodged by the rotating brush falls back or is brushed back onto the surface before it is gathered by the air flow.
A further disadvantage is that larger debris can be pushed along by the leading edge of the travelling head rather than being collected. This disadvantage is caused by the close proximity of the bottom of the leading edge to the surface being cleaned.
A further disadvantage is that larger debris that is collected can lodge in the ducts and block the vacuum cleaner.
Many vacuum cleaners are mains powered, and the manufacturers of mains powered vacuum cleaners will often seek to maximise the electrical and suction power of their vacuum cleaners in an attempt to increase their marketability. Typically, the opening of the travelling head is surrounded by a wall which permits a relatively small air flow into the travelling head. The air is forced to pass underneath the wall, through the underlying carpet or other floor covering, whereby to dislodge dirt and debris from between the fibres of the carpet. As impellers are typically 10 to 40% efficient in use and air is not particularly good at dislodging dust, dirt and debris, this is a relatively inefficient method of cleaning. In order to achieve higher impeller efficiencies, manufacturers have tended to develop faster spinning impellers creating higher suction. However, as it is air flow rather than suction which dislodges dirt and debris, such vacuum cleaners generally do not achieve improved dirt and debris collection efficiency. Manufacturers have therefore tended to quote electrical and suction power as a measurement of effectiveness of their appliances rather than cleaning efficiency.
It is also known to provide battery-powered vacuum cleaners. Battery-powered vacuum cleaners employing this traditional approach cannot provide the suction power of a mains powered vacuum cleaner without prejudicing the operating cycle of the vacuum cleaner, i.e. without unacceptably shortening the period between battery recharging, and therefore do not provide comparable cleaning performance.
It is an aim of the manufacturers of most domestic vacuum cleaners (mains powered and battery powered), that the travelling head has a height which allows the user to clean underneath chairs, cupboards and the like. The inventor considers a reasonable height limit to be 90 mm.
It will be understood that vacuum cleaners are not the only form of surface cleaning apparatus, and “carpet sweepers” are known which do not utilise suction. Carpet sweepers typically have a travelling head with an opening adjacent to the leading edge. A rotatable brush is mounted in the travelling head, the brush having bristles which project from the opening. The brush may be rotated by way of gearing connected to the wheels of the travelling head, so that movement of the travelling head across the surface being cleaned causes the brush to rotate. Alternatively, some carpet sweepers have a motor to rotate the brush. Carpet sweepers rely upon the mechanical dislodgement of dirt and debris from the surface being cleaned by the rotating brush. Only dirt and debris which is lifted from the surface and pushed into a dirt-collection chamber will be captured by the carpet sweeper, and some of the dirt and debris which is dislodged falls back onto the surface. Whilst the rotating brush generates air currents within the travelling head those air currents are incidental and do not significantly assist the cleaning operation, i.e. the air currents are turbulent and do not carry a significant amount of dirt and debris from the surface being cleaned and into the dirt-collection chamber.
Self-propelled or robotic vacuum cleaners are also known, and many have one or more rotating brushes to dislodge dirt and debris. The known robotic vacuum cleaners are substantially circular in plan view, which is necessary to reduce the likelihood that the vacuum cleaner will collide with, and perhaps become stuck by, articles of furniture and the like. However, the requirement to fit the componentry into the circular housing compromises the cleaning efficiency, with the shape of the air flow duct in particular having to be restricted to fit within the housing. On the other hand, most vacuum cleaners, and most carpet sweepers, are substantially rectangular in plan view, as this usually represents the most efficient shape in terms of packaging and performance.
Many prior art vacuum cleaners have a relatively long and tortuous path between the opening in the travelling head and the dirt-collection chamber. The intention is generally to maintain a high air speed through the travelling head so as to keep the dirt and debris entrained within the air flow. Also, a more tortuous path reduces the likelihood that dirt and debris will fall back out of the vacuum cleaner, particularly after the air flow has been stopped.